There has been an ever-increasing need for high-speed and accurate three-dimensional measurement systems. This is particularly true in the electronics manufacturing industry. One main drive behind this continuous need is miniaturization of electronics components or sub-assemblies. As components become smaller, need for controlling and monitoring manufacturing quality becomes greater. At the same time, trends toward high-throughput, automated manufacturing lines continue to grow at a fast rate. In turn, this requires non-contact measurement methods because tactile measuring systems are to slow, and at times, completely impractical. For this reason, non-contact optical techniques for measuring dimensions of parts or checking integrity of assemblies have become extremely popular.
Optical phase-shifting profilometry of the prior art is a well-known non-contact technique for constructing three-dimensional profiles of objects, as discussed by M. Halioua, U.S. Pat. No. 4,641,972, Halioua, M. and Liu, H.-C., “Optical Three-Dimensional Sensing by Phase Measuring Profilometry,” Opt. Lasers Eng., 11(3), 185-215 (1989) and J. E. Greivenkamp and J. H. Bruning, “Phase shifting interferometry,” in Optical Shop Testing, D. Malacara (ed.), Wiley, New York, 1992. The method has been adopted, used and documented in numerous applications in the past, such as disclosed in U.S. Pat. Nos. 6,639,685 to Gu, et al, 6,525,331 to Ngoi et al, 6,438,272 to Huang et al, 6,208,416 to Huntley et al, 5,471,303 to Ai et al, and “Adaptation of a Parallel Architecture Computer to Phase Shifted Moiré Interferometry,” Proc. SPIE, Vol. 728, 183-193 (1986). Referring to FIG. 1a, one prior art method involves obliquely projecting a varying-intensity pattern 10 using a projector 12 onto a surface under examination 14, and then viewing surface under examination 14 using imaging device 16, which may be a CCD camera. The image produced by imaging device 16 is digitally recorded by computer 18. An example of the varying intensity pattern 10 is shown in FIG. 1b. The projected pattern is then shifted (using a mechanical positioning system or suitable device) along a direction of intensity variation by a known fraction, for example, ¼ of the wavelength of the intensity waveform, and another image of surface under examination 14 is captured by imaging device 16 and recorded by computer 18. This process is repeated until the total amount of shift is equal to one whole wavelength of the intensity waveform, as shown in FIG. 1c. For example, if the amount of shift is ¼ of the wavelength, then altogether 4 images of surface under examination 14 will be captured. These images will correspond to 0, 90, 180, and 270-degree phase shifts of the intensity pattern. Similarly, if the amount of shift is ⅛th of the wavelength, then 8 images will be captured. For those who are not fully familiar with the art of phase-shifting profilometry, the term “wavelength” here refers to the distance that covers one complete cycle of intensity variation and NOT the wavelength of the illumination source that is used for projecting the varying intensity pattern 10. Once all images have been acquired, phase values of the projected intensity pattern are computed for each pixel using a phase calculation technique such as that reported in B. V. Dorrio and J. L. Fernandez, “Phase-evaluation methods in whole-field optical measurement techniques,” Measurement Science, 10, 33-55, 1999, and Asundi, A., “Fringe Analysis in Moiré Interferometry,” Proc. SPIE, Vol. 1554B, 472-480 (1991). These phase values are known as “wrapped” phases as they have a range of −π to π. These wrapped phases will then be further processed thru an “unwrapping” operation such as those described in Zebker and Lu, “Phase unwrapping algorithms for radar interfreometry: residue-cut, lease squares, and synthesis algorithms”, J. Opt. Soc. Am. A/Vol 15, No 3, March 1998, Herraez, Gdeisat, Burton and Lalor, “Robust, fast and effective two dimensional automatic phase unwrapping algorithm based on image decomposition”, Applied Optics, Vol 41, No 35, December 2002, Herraez, Burton, Lalor and Gdeisat “Fast two-dimensional phase-unwrapping algorithm based on sorting by reliability following a noncontinuous path”, Applied Optics, Vol 41, No 35, December 2002, Asundi and Wensen, “Fast phase-unwrapping algorithm based on a gray-scale mask and flood fill”, Applied Optics, Vol 37, No 23, August 1998. Any of these methods of the prior art will reveal height variations of surface under examination 14.
As explained above, phase-shifting profilometry requires multiple images of the scene to be captured. This repeating process of projecting a pattern onto a surface under examination and acquiring an image of that surface and shifting the pattern is too time-consuming for certain applications where speed of obtaining a three-dimensional profile is critical. Such applications include, for example, volumetric measurement of solder paste or flux deposits on printed circuit boards, and height measurement of solder ball or bumps. Attempts have been made to speed up the “project-acquire-shift” cycle by using solid-state projection devices such as LCD projectors as opposed to shifting the pattern using mechanical systems. However, these approaches are still too slow for applications requiring high speed or high throughput.
Another important issue is that calculation of wrapped phases, which directly influence accuracy of computing height values, is noticeably sensitive to a number of intensity values sampled at a point. Existing systems use a 3 or 4-step phase shift process, which implies that 3 or 4 images need to be captured. To improve accuracy of calculating wrapped phase values, many more samples are needed. Experiments have shown that for industrial applications, more than 8 intensity samples are needed. Moreover, computation of wrapped phases and subsequently, derivation of the height measurement at any point within a surface being examined, relies on obtaining well-illuminated images. This means that phase-shifted images should not be under-exposed (too dark) or over-exposed (too bright) so that there would be too little change from one phase-shifted image to a next phase shifted image at the same point in the scene. When the images are under or over-exposed, phase calculation will simply fail to yield a correct phase angle and consequently a height calculation will be incorrect. With scenes that contain objects of relatively uniform reflectivity, optimum imaging conditions may be found so that under or over-exposed situations will be minimized or eliminated. However, with most practical applications such as inspection of printed circuit boards or electronic wafers, a scene usually contains objects with substantially varying reflectivity from one part to another part. For example, a printed circuit board may contain solder paste, copper pads, solder mask, prints, copper traces and plastic electronic components of different color and reflectivity. In these situations, finding a compromise that works well for all parts of the scene can pose a serious challenge. Current methods do not address the issue of under or over-exposed images in a manner that lends itself to high-speed inspection.
A technique proposed in U.S. Pat. No. 5,646,733, to Bieman, and U.S. Pat. No. 6,522,777, to Paulsen et al uses a tri-linear imaging sensor for capturing images of three different phases at the same time. Basically, this technique uses an imaging device that has three rows of imaging cells (pixels) spaced from each other. The projected pattern and imaging optics are arranged such that each row of imaging cells will image a line-section of a scene that is illuminated with a known phase shift. For example, a first row of imaging cells will image a section of a scene illuminated by 0-degree phase-shift, a second row of imaging cells will image a section illuminated by 120-degree phase-shift and a third row of imaging cells will image a section illuminated by 240-degree phase-shift. The images from all three rows can be combined to compute a wrapped phase angle for all points along a line section of the scene. To construct a complete surface profile of an entire scene, the imaging head is moved to scan an area of interest and acquire a large number of profile slices that collectively describe the surface under examination. Although the invention works for its intended purpose, in practice its use is very limiting. A primary reason for this is that the invention of the prior art does not address the problem of under or over-exposed image sections that yield invalid height measurements, and more importantly a fixed number of imaging rows imply the limitation that there are only as many phase-shifted patterns as there are rows of imaging cells, in turn significantly limiting measuring capability of the system. In practice, a measuring resolution of optical phase-shifting is heavily dependant on the number of images taken to compute a phase angle, and with only three images, accuracy of the results may not be sufficient. Moreover, fixed spacing between rows of imaging cells implies that for a given wavelength of projected pattern, the imaging optics must be set up so that the imaging device will image the correct sections of the projected pattern. This in turn implies that design of imaging optics for the system will be dedicated by geometry of the imaging sensor and desired wavelength of a projected pattern. Hence, one has no control over spatial resolution of the system in a plane along a surface being examined. This is a serious practical limitation.
Another technique proposed in U.S. Pat. No. 6,040,910, to Wu et al, attempts to address the issue concerning under and over-exposed images. The technique of this prior art reference projects a number of references intensities onto an entire surface being examined and records resulting images that in turn are processed by a computer to compute an intensity amplitude for each point (pixel) within the scene so that all parts of the phase-shifted images are well-illuminated. Computed amplitudes are then used to control intensity of projected patterns whereby a temporal illumination system is used to dynamically construct and project the phase-shifted patterns. This technique offers a good solution for those cases where there is sufficient time to manipulate an illumination source and construct projected patterns on a scene-by-scene basis. However, for high-speed applications, this is not a practical solution as it is too slow.
A technique presented in U.S. Pat. No. 6,509,559, to Ulrich et al, describes a method of projection that uses binary patterns to construct a sinusoidal pattern on a surface being examined with a view towards improving contrast of acquired intensity values.